A gene that protected man from an ancient retrovirus that infected chimps and other primates millions of years ago may be one reason why our species is susceptible to HIV, said a study released Thursday. The gene appears to have been instrumental in protecting man against a four-million-year-old virus, called Pan troglodytes endogenous retrovirus, or PtERV1.
The virus is now extinct, but like most retroviruses, it inserts its DNA into the genome of organisms it infects, and it left its fingerprints all over the chimpanzee genome.
In fact, when the chimp genome was sequenced in 2005, it emerged that the largest difference between chimp and human DNA was the presence of PtERV1. Researchers at the Fred Hutchinson Cancer Research Center in Seattle, Washington, were intrigued by the disparity and speculated that the reason the virus had not left any markers in the human genome was because of an antiviral defense gene called TRIM5a.
"We know that PtERV1 infected chimps, gorillas and old world monkeys four million years ago but left no traces of having infected humans. Our theory is that this is because humans had this innate viral defense system," said Michael Emerman, a virologist at the center. To test their theory, Emerman and his team painstakingly reconstructed a small portion of the PtERV1 virus using DNA sequences from the chimp genome, and in laboratory experiments with human cells, matched it up with the human antiviral protein TRIM5a.
The TRIM5a neutralized the partial virus but when the investigators re-engineered the protein to make it more effective against HIV, a modern-day retrovirus, the changes inhibited the protein's ability to stop PtERV1 and vice versa. This suggests that the antiviral gene may only be good at fighting off one virus at a time, according to the authors of the study in the journal Science.
"TRIM5a may have served humans well millions of years ago, the antiviral protein does not seem to be good at defending against any of the retroviruses that currently infect humans, such as HIV-1," said Emerman. "In the end, this drove human evolution to be more susceptible to HIV," or human immunodeficiency virus.
The study does not have any implications for modern-day research into HIV but sheds some light on how ancient viruses have shaped our "viral defenses," he said.